Method for coating zinc-plated steel strips with aqueous formulations of acidic polymers

ABSTRACT

Continuous method of passivating galvanized steel strips with acidic aqueous formulations which comprise at least one water-soluble polymer comprising acidic groups.

The invention relates to a continuous method of passivating galvanizedsteel strips with acidic aqueous formulations which comprise at leastone water-soluble polymer comprising acidic groups.

The raw material used for producing sheetlike steel workpieces, such as,for example, automotive components, bodywork components, equipmentcasings, exterior architectural facings, ceiling panels or windowprofiles, presently comprises typically long steel strips which areproduced by hot rolling and/or cold rolling from steel slabs and whichare wound into coils for the purposes of storage and transportation. Thesteel strips may subsequently be divided up and processed to the desiredshaped parts.

The steel strips are generally protected from corrosion by means ofappropriate measures. This generally involves multistage operations. Ina first step the steel strips are typically coated with zinc or withzinc alloys. The action of zinc is based on the one hand on the factthat it is baser than steel and thus initially undergoes corrosionitself. The steel surface remains intact as long as it is still coveredcontinuously with zinc. Moreover, in the presence of atmospheric oxygen,a thin oxide layer forms on the surface of Zn or of Zn alloys, and,depending on the external conditions, slows down, to a greater or lesserextent, the corrosive attack on the underlying metal.

In order to boost the protective effect of such an oxide layer, the Znsurfaces are generally subjected town additional passivating treatment.In the course of such a treatment, some of the metal to be protecteddissolves and is incorporated at least partly into a film on the metalsurface. Instead of the term “passivation coat” the terms “conversioncoat”, “aftertreatment coat” or “pretreatment coat” are also usedsynonymously.

The performance of such passivation by treatment of the galvanized steelsurface with acidic Cr(VI) and/or Cr(III) solutions is known.Increasingly, however, use is also being made for this purpose ofchromium-free formulations, examples being phosphate formulations orelse formulations which comprise various polymers.

One important class of polymers which can be used in formulations forpassivating treatments comprises strongly acidic, water-solublepolymers, such as, for example, polyacrylic acid or copolymers ofacrylic acid with other monomers, more particularly with other acidicmonomers such as vinylphosphonic acid, maleic acid or itaconic acid. Theuse of polymers of this kind for passivation is disclosed in, forexample, WO 2004/074372, WO 2005/042801, WO 2006/021308, 2006/021309, WO2006/134116, WO 2006/134117 or WO 2006/134118. The polymers in each caseform the main constituent of the formulations—more particularly, furtheracids, such as phosphoric acid, for example, are present in theformulation in small amounts, if at all. In the course of thepassivation the acid groups of the polymer detach zinc and also,possibly, further metals from the surface, with formation of hydrogen.The zinc ions that have formed on the one hand are redeposited on thesurface, but may also crosslink the polymer layer by forming complexeswith the acid groups of the polymer and also, possibly, with otherfunctional groups of the polymer, thus producing a very densepassivation coat.

The passivation of galvanized steel strips is typically performed bymeans of a continuous method, as is shown diagrammatically in FIG. 1). Agalvanized steel strip (1) is moved by means of drive rollers (notshown) in direction (2). By means of a spraying station (3), theformulation for passivating is sprayed onto the steel strip and forms awet film (7). Two squeeze rolls, (4) and (4′), squeeze off excesstreatment solution. The result is a thin, wet film (8), which finally isdried in a dryer (6). This gives a galvanized steel strip withpassivation coat (9). The formulation squeezed off can be collected (5)and used again for coating.

When passivation is carried out using formulations of strongly acidic,water-soluble polymers, this method exhibits a series of disadvantages:In order to achieve a satisfactory passivation outcome, the acidicpolymers ought to be used at concentrations of at least 10% by weight,based on the formulation employed. At lower concentrations, the pH ofthe formulations is too high, with the consequence that the zinc surfacecannot be attacked by acid with sufficient speed, and the resultingpassivation coats are worse in quality. Furthermore, after theapplication of the passivation coat, acidic groups which have not yetreacted may react in the course of time, possibly leading to a whiteningof the passivation coat (referred to as “staining”). Such discolorationis highly undesirable.

Formulations with more than 10% by weight of strongly acidic,water-soluble polymers have a much higher viscosity than formulationswhich comprise predominantly low molecular mass components, such astypical formulations based on Cr(VI), Cr(III) or phosphoric acid, forexample. When high-viscosity formulations of this kind are used, abuild-up (8) is formed ahead of the squeeze roll (FIG. 1, (4)).

The development of a build-up, however, is associated with a series ofdisadvantages: Since the dwell time of individual polymer molecules inthe build-up is not constant, but instead is randomly distributed, thepassivation reaction in the build-up proceeds uncontrolledly. As aresult of the reaction with the zinc surface, zinc increasinglyaccumulates in the build-up. Since zinc, by forming complexes with theacid groups of the polymer, increasingly crosslinks the polymerformulation, there is a change in the viscosity of the formulation inthe build-up. Moreover, the formulation squeezed off becomescontaminated with zinc, and as a result, ultimately, it can no longer beused. Because of the viscosity in the build-up, there is also,increasingly, a risk of air bubbles forming which, on passing throughthe squeeze rolls, lead to some sites on the strip's surface no longerbeing covered with a passivation coat. On the basis of the conditionsstated, it is very difficult to apply thin, homogeneous passivationcoats which ensure effective protection from corrosion. With the processtechnology depicted, the dry film thickness of the passivation coatsformed is, as a general rule, more than 1 μm, a thickness which canhardly be reduced even by means of harder rolls.

Within the field of coil coating with paints, multiple-roll systems forapplication ere known, with 2, 3 or more rolls for applying the paint(see, for example, “Coil coating” in Römpp Lexikon Lacke andDruckfarben, page 55, Georg Thieme Verlag Stuttgart, New York, 1998).Multiple-roll systems of thiskind, however, are very complicated andexpensive.

In the field of paper coating there are devices known with which papercoating compositions, such as adhesive layers, for example, are appliedto a coating roll and, even prior to contacting with the paper, excessmaterial is doctored off using a suitable doctor means. Reference may bemade, for example, to DE 37 35 889 A1, DE 198 00 955 A1 or U.S. Pat. No.2,970,564. The use of techniques of this kind for passivating galvanizedsteel strips with formulations of strongly acidic polymers was unknownto date.

It was an object of the invention to provide a continuous method ofpassivating galvanized steel strips with aqueous formulations ofstrongly acidic, water-soluble polymers, in which even formulations withat least 10% by weight of polymers can be satisfactorily processed tothin passivation coats. The process technology involved in this methodshould be extremely simple, and should allow even existing plant forpassivating galvanized steel strips to be easily retrofitted.

Surprisingly it has been found that technologies known from the field ofthe coating of paper and cardboard webs are also suitable for theapplication of passivation coats to galvanized steel strips. By means ofthese techhologies it is possible to apply even very thin passivationcoats with high quality.

Found accordingly has been a continuous method of passivating galvanizedsteel strips with an acidic aqueous formulation comprising at least onewater-soluble polymer X comprising acidic groups,

-   -   the polymer X having at least 0.6 mol of acid groups/100 g of        polymer,    -   the pH of the formulation being not more than 5, and    -   the amount of all the polymers X together being 10% to 30% by        weight, based on the amount of all the components of the        formulation,        and the galvanized steel strip being moved in its longitudinal        direction through an adjustable gap (12) between two opposing        rolls (4), (4′) which rotate in the direction of travel (2) of        the steel strip, and wherein    -   the acidic aqueous formulation is applied to at least one of the        two rolls using an applicator means (10), and between the        applicator means (10) and the gap (12) in the direction of        rotation of the roll there is at least one doctor means (11)        with which, by removal of excess formulation, it is possible to        set the amount of the aqueous formulation per unit area of the        roll,    -   the film located on the roll, on contact with the galvanized        surface of the steel strip, is transferred wholly or partly to        the galvanized surface, and    -   the wet film is dried in a drying means (6) disposed downstream        of the pair of rolls in the direction of travel (2) of the        strip.

Index to the figures:

FIG. 1: Diagrammatic representation of a prior-art passivating method

FIG. 2: Diagrammatic representation of the passivating method of theinvention for single-sided passivation

FIG. 3: Diagrammatic representation of the passivating method of theinvention for double-sided passivation

Details of the invention now follow:

Passivation by means of the methdd of the invention is accomplishedusing an acidic aqueous formulation which comprises at least onewater-soluble polymer X comprising acidic groups. The polymers Xemployed may be homopolyme'rs or copolymers. It will be appreciated thatmixtures of two or more different polymers X can also be used.

As solvents the formulation comprises preferably just water. It mayadditionally comprise water-miscible organic solvents in small amounts.Examples comprise monoalcohols such as methanol, ethanol or propanol,higher alcohols such as ethylene glycol or polyether polyols, etheralcohols such as butyl glycol or methoxypropanol, andN-methylpyrrolidone. As a general rule, however, the amount of water isat least 80%, preferably at least 90%, and very preferably at least 95%by weight. The figures are based in each case on the total amount of allthe solvents.

In accordance with the invention the polymers X employed have at least0.6 mol of acid groups/100 g of polymer. This quantity figure is basedon the free acid groups. Preferably the polymers have at least 0.9 molof acid groups/100 g, more preferably at least 1 mol/100 g, and verypreferably at least 1.2 mol/100 g.

The formulation employed in accordance with the invention comprises 10%to 30%, preferably 12% to 30%, more preferably 15% to 30%, and verypreferably 18% to 28% by weight of the polymers X, based on the amountof all the components of the formulation (including the solvents).

The term “water-soluble” for the purposes of this invention is intendedto denote that the polymer or polymers X employed are homogeneouslywater-soluble. Aqueous dispersions of crosslinked polymer particles ofinherently water-insoluble polymers as polymer X are not part of thescope of this invention. The polymers X employed ought preferably to beinfinitely miscible with water. They must at least, however, bewater-soluble to an extent such that it is possible to attain the statedconcentrations in the formulation.

The formulation employed has a pH of not more than 5, more particularlya pH of 0.5 to 5, preferably 1.5 to 3.5. Naturally, the pH of thepreparation depends on the nature and concentration of the polymers Xused in accordance with the invention. It may additionally be influencedby further basic or acidic components in the formulation.

Accordingly, as well as the acidic polymers, the formulation employedmay further comprise organic or inorganic acids or mixtures thereof.There is no limit on the selection of such acid, provided that nonegative effects arise together with the other components of theformulation. The skilled person will make an appropriate selection.Examples of suitable acids comprise phosphoric acid, phosphonic acid ororganic phosphonic acids such as 1-hydroxyethane-1,1-diphosphonic acid(HEDP), 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC),aminotri(methylenephosphonic acid) (ATMP),ethylenediaminetetra(methylenephosphonic acid) (EDTMP) ordiethylenetriaminepenta(methylenephosphonic acid) (DTPMP), sulfonicacids such as methanesulfonic acid, amidosulfonic acid,p-toluenesulfonic acid, m-nitrobenzenesulfonic acid, and derivativesthereof, nitric acid, formic acid or acetic acid. Preference is given tophosphorus acids such as H₃PO₄, phosphonic acid, the stated organicphosphonic acids, HNO₃ or methanesulfonic acid. With preference the acidin question may be H₃PO₄ or another phosphorus acid.

The acidity of the formulation employed ought, however, to be producedsubstantially by the acid groups of the polymer. The amount ofadditional acids, besides the polymers X, in the formulation oughttherefore not to exceed, in general, 50% by weight, relative to theamount of all the polymers X in the formulation together. Preferably30%, more preferably 20%, and very preferably 10% by weight ought not tobe exceeded. In a second, particularly preferred embodiment of theinvention there are no additional acids.

The acid groups of the polymer X ought preferably to be in the form offree acid groups. However, to a small extent, they may also beneutralized by means of bases, such as ammonia, amines, amino alcoholsor alkali metal hydroxides, for example. A partial neutralization ofthis kind may be performed for the purpose of pH adaptation.Alternatively it may, come about automatically in the course of thepreparation of the polymer. The skilled worker, for example, is awarethat it can be necessary, in the course of the preparation of polymersrich in acid groups, to neutralize some of the acid groups in order toforce incorporation of the monomers into the polymer.

In no way, however, should the degree of neutralization be too high toensure effective attack of acid on the zinc surface. As a general rule,therefore, not more than 25 mol % of the acid groups present in thepolymer X should be neutralized, preferably not more than 20 mol %, andmore preferably not more than 12 mol %.

The acid groups of the polymers X are selected in general from the groupconsisting of carboxylic groups, sulfonic add groups, phosphoric orphosphonic acid groups. The groups in question are preferably carboxylgroups, phosphoric or phosphonic acid groups. The polymer X employed ismore preferably a copolymer of at least two different acid-functionalmonomers.

For the performance of the invention it is particularly preferred to usehomopolymers or copolymers which comprise (meth)acrylic acid units.

In one particularly preferred embodiment of the invention, the polymer Xcomprises one or more water-soluble copolymers X1 of (meth)acrylic acidunits (A) and different monoethylenically unsaturated monomers withacidic groups (B). Optionally, furthermore, (meth)acrylic esters (C)containing OH groups, and/or further monomers (D) may be present asstructural units. Over and above these there are no further monomerspresent.

The amount of (meth)acrylic acid (A) in the copolymer X1 is 30% to 90%,preferably 40% to 80%, and more preferably 50% to 70% by weight, thisfigure being based on the sum of all the monomers in the polymer.

The monomer (B) is at least one monoethylenically unsaturated monomerwhich is different from (A) but copolymerizable with (A) and whichcontains one or more acidic groups. It will be appreciated that two ormore different monomers (B) can also be used.

The acidic groups in question may preferably comprise a group selectedfrom the group consisting of carboxyl groups, phosphoric acid groups,phosphonic acid groups or sulfonic acid groups. The group in question ispreferably selected from the group consisting of carboxyl groups,phosphoric add groups or phosphonic acid groups.

Examples of monomers of this kind comprise crotonic acid, vinylaceticacid, C₁-C₄ monoesters of monoethylenically unsaturated dicarboxylicacids, styrenesulfonic acid, vinylsulfonic acid, allylsulfonic acid,2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), vinylphosphonicacid, monovinyl phosphate, maleic acid, fumaric acid or itaconic acid.

The amount of the monomers (B) in the copolymer X1 is 10% to 70%,preferably 20% to 60%, and more preferably 30% to 50% by weight, basedin each case on the sum of all the monomers in the polymer.

In one preferred embodiment of the invention the monomers (B) aremonoethylenically unsaturated dicarboxylic acids having 4 to 7 carbonatoms (B1) and/or monoethylenically unsaturated phosphoric and/orphosphonic acids (B2).

Examples of monomers (B1) comprise maleic acid, fumaric acid,methylfumaric acid, methylmaleic acid, dimethylmaleic acid,methylenemalonic acid or itaconic acid. The monomers may if desired alsobe used in the form of the corresponding cyclic anhydrides. Preferenceis given to maleic acid, fumaric acid, and itaconic acid, particularpreference to maleic acid and/or maleic anhydride.

Examples of monomers (B2) comprise vinylphosphonic acid, monovinylphosphate, allylphosphonic acid, monoallyl phosphate,3-butenylphosphonic acid, mono-3-butenyl phosphate, mono-4-vinyloxybutylphosphate, phosphonoxyethyl acrylate, phosphonoxyethyl methacrylate,mono-2-hydroxy-3-vinyloxypropylphosphate,mono-1-phosphonoxymethyl-2-vinyloxyethyl phosphate,mono-3-allyloxy-2-hydroxypropyl phosphate,mono-2-allylox-1-phosphonoxymethylethyl phosphate,2-hydroxy-4-vinyloxymethyl-1,3,2-dioxaphosphole, and2-hydroxy-4-allyloxymethyl-1,3,2-dioxaphosphole. Preferably the monomeris vinylphosphonic acid, monovinyl phosphate or allylphosphonic acid,particular preference being given to vinylphosphonic acid.

The copolymer X1 may further optionally comprise at least one(meth)acrylic ester (C) having OH groups. The monomers in question arepreferably monohydroxy (meth)acrylic esters.

The monomers (C) preferably comprise at least one (meth)acrylic ester ofthe general formula H₂C═CHR¹—COOR², where R¹, in a manner known inprinciple, stands for H or methyl and where R² is selected from thegroup consisting of R^(2a), R^(2b) or R^(2c).

The radicals R^(2a) are radicals of the general formula —(R³—O—)_(n)—H.In this formula n stands for a natural number from 2 to 40. Preferably nstands for 2 to 20 and more preferably for 2 to 10. The radicals R³, ineach case independently of one another, stand for a divalent,straight-chain or branched alkyl radical having 2 to 4 C atoms. Examplescomprise, more particularly, 1,2-ethylene radicals, 1,2-propyleneradicals, 1,2-butylene radicals, and 1,4-butylene radicals. It will beappreciated that they may also comprise mixtures of different radicals.Preference is given to 1,2-ethylene and/or 1,2-propylene radicals.Particular preference is given exclusively to 1,2-ethylene radicals.Further preference is given to radicals (R^(2a)) which feature not only1,2-ethylene but also 1,2-propylene radicals, the amount of the ethyleneradicals being at least 50%, preferably at least 70%, and morepreferably at least 80%, based on the total number of all the radicalsR³. Examples of radicals R^(2a) comprise —CH₂—CH₂—O—CH₂—CH₂—O—CH₂—CH₃,—CH₂—CH(CH₃)—O—CH₂—CH(CH₃)OH, —CH₂—CH(CH₃)—O—CH(CH₃)—CH₂OH,—CH(CH₃)—CH₂—O—CH(CH₃)—CH₂OH or —CH(CH₃)—CH₂—O—CH₂—CH(CH₃)OH.

The radicals R^(2b) are radicals of the general formula —R⁴—(OH)_(m). Inthis formula m is a natural number from 1 to 6, preferably 1 to 4, morepreferably 1 to 3, and, for example, 1 or 2. The radical R⁴ stands foran (m+1)-valent, straight-chain or branched alkyl radical having 2 to 10C atoms, preferably 2 to 6 C atoms, and more preferably 2 to 4 C atoms.

The alkyl radical is substituted by at least one OH group, with theproviso that there is not more than one OH group X per C atom in R⁴.Examples of suitable radicals R^(2b) with OH groups comprise linearradicals of the general formula —(CH₂)_(m)—OH such as —CH₂—CH₂—OH,—CH₂—CH₂—CH₂—OH, —CH₂—CH₂—CH₂—CH₂—OH or —CH₂—CH₂—CH₂—CH₂—CH₂—CH₂—OH.

Particularly preferred radicals R^(2b) for performing the invention areradicals selected from the group consisting of —CH₂—CH₂—OH,—CH₂—CH₂—CH₂—OH, —CH₂—CH₂—CH₂—CH₂—OH, —CH₂—CH(CH₃)—OH, —CH(CH₃)—CH₂—OHor —CH₂—CH(OH)—CH₂—OH.

In a further, preferred embodiment of the invention at least one of theradicals R^(2b) is a branched alkyl radical of the general formula—R⁵—CH(R⁶)OH. In this formula R⁵ and R⁶ each stand for a linear orbranched alkyl radical having 1 to 8 C atoms, preferably 1 to 6 C atoms,and more preferably 1 to 4 C atoms, with the proviso that the sum of theC atoms in R⁵ and R⁶ is not more than 9. Preferably R⁵ and R⁶ are eachlinear alkyl groups. More preferably R⁶ is a methyl group. The radicalin question may be, for example, —CH₂—CH(CH₃)—OH. In the case ofbranched (meth)acrylic esters of this kind, the tendency of the OH groupto form further ester bonds with other COOH-containing monomers issignificantly reduced. Very particular preference is given to—CH₂—CH(CH₃)—OH and/or —CH(CH₃)—CH₂—OH, more particularly to a mixtureof both radicals. (Meth)acrylic esters with such radicals may beobtained in a simple way, as for example by esterification of(meth)acrylic acid with 1,2-propylene glycol.

The radicals R^(2c) are monosaccharide or oligosaccharide radicals,preferably monosaccharide radicals. The saccharides in question may inprinciple be all kinds of saccharides. With preference it is possible touse radicals derived from pentoses and hexoses, more particularly fromhexoses. Examples of suitable monosaccharides comprise glucose, mannose,galactose, fructose or ribose. With preference it is possible to useradicals derived from glucose. The radicals in question may also bederivatives of the saccharides, as for example products originating fromthe saccharides through reduction or oxidation. The saccharides inquestion may more particularly be sugar acids such as gluconic acid, forexample.

The amount of the monomers (C) in the copolymer X1 is 0% to 40%,preferably 1% to 30% by weight.

Besides the monomers (A), (B), (C), and, if used, (D), it is possibleoptionally to use 0% to 30% by weight of at least one furtherethylenically unsaturated monomer (D), different from (A), (B), and (C).Over and above these there are no other monomers used.

The monomers (D) serve to fine-tune the properties of the copolymer X1.It will be appreciated that two or more different monomers (D) can alsobe used. They are selected by the skilled worker in accordance with thedesired properties of the copolymer, and additionally on the conditionthat they must be copolymerizable with the monomers (A), (B), and (C).

The monomers in question are preferably monoethylenically unsaturatedmonomers. In special cases, however, small amounts of monomers with twoor more polymerizable groups may also be used. By this, means it ispossible for the copolymer to be crosslinked to a small extent.

Examples of suitable monomers (D) comprise, in particular, aliphaticalkyl esters of (meth)acrylic acid, such as methyl (meth)acrylate, ethyl(meth)acrylate, butyl (meth)acrylate, or 2-ethylhexyl (meth)acrylate.Additionally suitable are vinyl or allyl ethers such as methyl vinylether, ethyl vinyl ether, propyl vinyl ether, 2-ethylhexyl vinyl ether,vinyl cyclohexyl ether, vinyl 4-hydroxybutyl ether, decyl vinyl ether,2-(diethylamino)ethyl vinyl ether, 2-(di-n-butylamino)ethyl vinyl etheror methyl diglycol vinyl ether, and/or the corresponding allylcompounds. It is likewise possible to employ vinyl esters, such as vinylacetate or vinyl propionate, for example. It is also possible to employbasic comonomers, such as acrylamide and alkyl-substituted acrylamides,for example.

Examples of crosslinking monomers comprise molecules having two or moreethylenically unsaturated groups, examples being di(meth)acrylates suchas ethylene glycol di(meth)acrylate or butane-1,4-diol di(meth)acrylateor poly(meth)acrylates such as trimethylolpropane tri(meth)acrylate orelse di(meth)acrylates of oligoalkylene or polyalkylene glycols, such asdi-, tri- or tetraethylene glycol di(meth)acrylate. Further examplescomprise vinyl(meth)acrylate or butanediol divinyl ether.

The amount of all the monomers (D) used, together, is 0% to 30% byweight, based on the total amount of the monomers used. Preferably theamount is 0% to 20% by weight, more preferably 0% to 10%. Ifcrosslinking monomers (D) are present, their amount ought as a generalrule not to exceed 5%, preferably 2% by weight, based on the totalamount of all the monomers used for the method. It can, for example, be10 ppm to 1% by weight.

In a first preferred embodiment of the invention the copolymer X1comprises, besides (A), at least one monomer (B). Preferably there areno further monomers (C) or (D) besides the monomers (A) and (B). Theamount of (A) in the case of this embodiment is preferably 60% to 90% byweight, and the amount of (B) 10% to 40% by weight. With particularpreference, in the case of this embodiment, the copolymer X1 is acopolymer of acrylic add and maleic acid or of acrylic acid and itaconicadd in the aforementioned amounts.

In a second preferred embodiment of the invention the copolymer X1comprises, besides (A), at least one monomer (B1) and at least onemonomer (B2). Furthermore, with particular preference, there are nofurther monomers (D) besides the monomers (A), (B1), and (B2). Theamount of (A) in the case of this embodiment is preferably 50% to 90% byweight, the amount of (B1) 5% to 45% by weight, the amount of (B2) 5% to45% by weight, and the amount of (D) 0% to 20% by weight. Withparticular preference the copolymer is a copolymer X1 of acrylic acid,maleic acid, and vinylphosphonic acid in the aforementioned amounts.

In a third preferred embodiment, of the invention the copolymer X1comprises at least one monomer (B2) and at least one monomer (C) besides(A). The amount of (A) in the case of this embodiment is preferably 20%to 60% by weight, the amount of (B2) 20% to 60% by weight, the amount of(C) 1% to 40% by weight, and the amount of (D) 0% to 20% by weight. Withparticular preference the copolymer in question is a copolymer X1 ofacrylic acid, vinylphosphonic acid, and hydroxyethyl acrylate and/orhydroxypropyl acrylate.

The preparation of the polymers X may take place in accordance withmethods that are known to the skilled worker. Preferably the copolymersare prepared by free-radical addition polymerization of the statedcomponents (A), (B), and, optionally, (C) and/or (D) in aqueoussolution. Details concerning the conduct of a free-radical additionpolymerization are known to the skilled worker. Preparation processesfor the copolymers X1 are described in, for example, WO 2006/021308 orWO 2006/134116 at page 9 line 38 to page 13 line 24.

The synthesized copolymers X1 can be isolated from the aqueous, solutionby means of typical techniques known to the skilled worker, as forexample by evaporating down the solution, spray drying, freeze drying orprecipitating. Preferably, however, the copolymers X1 after thepolymerization are not isolated at all from the aqueous solution;instead, the resulting solutions of the copolymers—following addition offurther additives if desired—are used as they are for the method of theinvention. In order to facilitate such direct further use the amount ofthe aqueous solvent used for the polymerization ought from the start tobe calculated such that the concentration of the polymer in the solventis suitable for the application. It is also possible first to prepare aconcentrate which only when in situ is diluted with water or,optionally, other solvent mixtures to the desired concentration.

The molecular weight M_(w) (weight average) of the polymers X and/orcopolymers X1 used for the method of the invention is specified by theskilled worker in accordance with the desired application. It ispossible, for example, to use polymers having a molecular weight M_(w)of 3000 to 1 000 000 g/mol. Polymers having proven appropriate includemore particularly polymers with 5000 g/mol to 500 000 g/mol, preferably10 000 g/mol to 250.000 g/mol, more preferably 15 000 to 100 000 g/mol,and very preferably 20 000 to 75 000 g/mol.

The formulation employed may optionally comprise further componentsabove and beyond the polymer X and the stated acid.

The formulation may, for example, optionally comprise metal ions ormetal compounds. If metal ions or metal compounds are to be present,however, the formulations in question should preferably not comprisechromium compounds. Moreover, there ought preferably to be no metalfluorides or complex metal fluorides present. In other words,preferably, the passivation of the invention is a chromium-freepassivation, more preferably a chromium-free and fluoride-freepassivation.

The metal ions may be metal ions selected from the group consisting ofZn²⁺, Mg²⁺, Ca²⁺ or Al³⁺. Preference is given to Zn²⁺ or Me, andespecial preference to Zn²⁺. Besides these the preparation preferablycomprises no further metal ions. The ions may take the form of hydratedmetal ions, or else may take the form of dissolved compounds—complexcompounds, for example. More particularly the ions may have complexbonds to the acidic groups of the polymer.

If present, the amount of the metal ions from the group consisting ofZn²⁺, Mg²⁺, Ca²⁺ or Al³⁺ is 0.01% to 20%, preferably 0.5% to 18%, andmore preferably 1% to 15% by weight, based in each case on the total,amount of all the polymers X in the formulation.

The formulation may further comprise at least one dissolved phosphateion. The ion in question may comprise all kinds of phosphate ions. Thespecies in question may be, for example, orthophosphates ordiphosphates. For the skilled worker it is clear that in aqueoussolution, depending on pH and concentration, there may be an equilibriumbetween the different dissociation states of the ions. Moreover, theformulation may comprise methanesulfonate ions.

Where phosphate ions are present, the metal ions and phosphate ions maybe used preferably in the form of soluble salts comprising both ions.Examples of such compounds comprise Zn₃(PO₄)₂, ZnH₂PO₄, Mg₃(PO₄)₂ orCa(H₂PO₄)₂, and corresponding hydrates thereof.

The metal ions and phosphate ions may alternatively be added separatelyfrom one another. For example, the metal ions can be used in the form ofthe corresponding nitrates, alkanesulfonates or carboxylates, acetatesfor example, and the phosphates in the form of phosphoric acid. It isalso possible to use insoluble or low-solubility compounds, such as thecorresponding carbonates, oxides, oxide hydrates or hydroxides, forexample, which are dissolved under the influence of add.

Similarly, metal ions and methanesulfonate ions can be used together as,metal salts of methanesulfonic acid, such as Zn(CH₃SO₃)₂, for example,or else separately in the form of other metal salts and methanesulfonicacid.

The amount of the phosphate ions and/or methanesulfonate ions in theformulation is specified by the skilled worker in accordance with thedesired properties of the formulation. Where they are present, theiramount is generally 0.01% to 20%, preferably 0.5% to 25%, morepreferably 1% to 25%, by weight, calculated in each case asorthophosphoric acid and based in each case on the polymers X.

The formulation may optionally further comprise at least one waxdispersed in the formulation. It will be appreciated that mixtures ofdifferent waxes can also be used. The term “wax” here comprises not onlywax itself but also auxiliaries that may be used to form a waxdispersion. The skilled worker is aware of waxes for use in aqueousdispersions, and will make an appropriate selection. The waxes may, forexample, be montan waxes, polyethylene waxes, waxes based on oxidizedpolyethylene, based on fluorinated polyethylene such as PTFE or otherpolymers based on C, H, and F. The term “polyethylene” is also intendedto comprise copolymers of ethylene and other monomers, more particularlyof other olefins such as propylene, for example. Ethylene copolymers ofthis kind preferably comprise at least 65% by weight of ethylene.Through the addition of waxes it is possible advantageously to lower thefriction of the surface with the surface of the tools used for shaping.

The amount of optionally employed waxes is determined by the skilledworker in accordance with the desired properties of the passivationcoat. An amount which has proven appropriate is in general from 0.01% to70%, preferably 0.5% to 25%, and more preferably 1% to 10%, by weight,based in each case on the polymer X.

Further components which can be used optionally for the formulationcomprise surface-active compounds, corrosion inhibitors, complexingagents, typical electroplating assistants, or else other polymers to bedistinguished from the polymers X used in accordance with the invention.

The skilled worker makes an appropriate selection from the optionalcomponents that are possible in principle, and with respect to theiramounts as: well, in accordance with the desired application. As ageneral rule, however, the amount of optional components alongside thepolymer X ought not to be more than 20%, preferably not more than 10%,and more preferably not more than 5% by weight, based on the polymers X.

The formulations for use in accordance with the invention can beobtained by simple mixing of the components. If waxes are used they arepreferably first dispersed separately in water and, in the form of adispersion, mixed with the other components. Wax dispersions of thiskind are also available commercially.

The method of the invention is performed using galvanized steel stripswhich are passivated on a coil coating line by means of a continuousprocess.

Galvanized steel strips typically have a thickness of 0.2 to 3 mm and awidth of 0.5 to 2.5 m. Galvanized steel strips are availablecommercially for a very wide variety of applications. The skilled workerselects a suitable steel strip in accordance with the desired end use.

The epithet “galvanized” also applies, of course, to steel strips whichhave been coated with Zn alloys. These may be hot dip galvanized orelectrolytically galvanized steel strips. Zn alloys for coating steelare known to the skilled worker. Depending on the desired endapplication, the skilled worker selects the nature and amount ofalloying constituents. Typical constituents of zinc alloys comprise moreparticularly Al, Mg, Pb, Si, Mg, Sn, Cu or Cd, preferably Al or Mg. Thealloys in question may also be Al Zn alloys in which Al and Zn arepresent in approximately the same amount. The coatings in question maybe largely homogeneous coatings or else coatings which exhibitconcentration gradients. The alloys may with further preference be Zn Mgalloys. In this case the steel may be steel coated with a Zn Mg alloy,hot dip galvanized steel for example, or may be galvanized steel whichhas additionally been vapor-coated with. Mg. This may produce a Zn/Mgalloy at the surface.

The strips may be galvanized on one or both sides. In the case of stripsgalvanized on both sides, the method of the invention can be used topassivate both sides or else only one side.

The method of the invention is performed using a coil coating line. Thesection of the line in which the strip is coated is showndiagrammatically in FIG. 2. Beyond the sections shown, the coil coatingline of course comprises further typical components, such as a means ofunwinding the raw steel strip from coil, a means of winding the coatedstrip, and a drive means for the strip. It may further optionallycomprise additional typical components, such as cleaning stations, forexample. The steel strips may be run through the line at a speed, forexample, of 30 to 200 m/s, preferably 50 to 150 m/s.

To effect coating, the line that is used comprises two mutually opposedrolls (4) and (4′). Between the two rolls (4), (4′) there remains a gap(12) which is adjustable in its width. For this purpose, one roll orelse both rolls can be provided with an appropriate mechanism foradjusting the roll position. For the performance of the method, thegalvanized steel strip is moved in its longitudinal direction throughthe adjustable gap (12) between the rolls (4), (4′). The rolls rotate inthe direction of travel (2) of the steel strip.

The rolls (4), (4′) may, in a way which is known in principle, comprisea steel core bearing on its outer face a polymeric coating. A polymericcoating may be composed, for example, of EPDM rubber and may be abradedif required. Rolls which have proven particularly appropriate for theperformance of the method of the invention are those having a relativelyhard surface, examples being those having a Shore A hardness of at least60, preferably at least 70, and more preferably at least 80. By way ofexample it is possible to use a roll of 90 to 98 Shore A. The surfaceroughness of the rolls can be chosen by the skilled worker in accordancewith the desired outcome. The drive of the rolls ought preferably to becapable of regulation independently from the drive of the steel strip.In this way the rolls can also move at a different speed from the strip,thereby allowing a certain slip to be set between the steel strip andthe rolls (4), (4′). The adjustability of the gap (12) between the tworolls allows the pressure exerted by the rolls to be varied.

In accordance with the invention, for passivation, the formulationdescribed is applied using a suitable applicator means (10) to at leastone of the two application rolls (4) and/or (4′). FIG. 2 showsdiagrammatically the case where only one side is passivated. Of course,application may also take place to both rolls, thereby allowing bothsides of the galvanized steel strip, to be passivated. In that case thesecond roll (4′) as well has an applicator means (10′). This is showndiagrammatically in FIG. 3.

There is no restriction here on the nature of the applicator means (10)and (10′). For example, the formulation can be applied to the roll byspraying, with the aid of appropriate nozzles, or transferred to theroll using a pouring means, a pouring gap. Furthermore, the formulationcan also be transferred to the roll via an open application chamber.Suitable constructions of open application chambers are known to theskilled worker from the field of the coating of paper or cardboard webs,and are shown in, for example, U.S. Pat. No. 2,970,564, DE 34 17 487 A1or DE 37 35 889 A1. The applicator may also comprise a roll applicatormechanism in which the formulation is taken from a reservoir vessel bymeans of a suitable Scoop roll and is transferred, directly or viafurther rolls, to the application rolls (4), (4′). Preference is givento methods wherein the formulation is applied directly to the rolls (4),(4′) and not transferred indirectly via further rolls from the reservoirvessel to the application rolls (4), (4′).

In accordance with the invention the coil coating line used comprises,between the applicator means (10) and the gap (12) in the direction ofrotation of the roll (4) and/or (4′), at least one doctor means (11)and/or (11′) with which, by removal of excess formulation, it ispossible to set the amount of the aqueous formulation per unit area ofthe roll. In this case, at the same time, a more uniform film on theroll is obtained. The excess formulation removed can be returned to thecoating operation. Since at this stage of the method the formulation isnot yet in contact with the zinc surface, the returned formulation isalso not contaminated with detached zinc, and is therefore unchanged inits rheological properties. This is a great advantage, in contrast tothe above-described prior-art passivating methods. The formulation canbe returned by means, for example, of excess formulation removed fromthe doctor means (11) simply running back into the applicator means, or,for example, by the formulation being collected and conveyed back intothe reservoir container, by means of pumps, for example. The amount offormulation transferred to the roll (4) and/or (4′) per unit area may ofcourse be controlled not only by the doctor means, but also, if desired,by the setting of the applicator means (10) and also by furtherparameters, such as the speed of the application rolls (4) and/or (4′),for example.

As the doctor means it is possible in principle to use the doctor meansthat are known to the skilled worker. The doctor means may be composedof metal, plastic, coated metal, glass or ceramic.

The doctor means (11) may for example be a doctor knife or a doctorblade. The blade may be a smooth one or may also have notches on itssurface. The amount of the formulation which is intended to remain onthe roll (4) and/or (4′) may be accomplished in a typical way by thesetting of the gap between the doctor knife and the roll (4) and/or(4′).

In one preferred embodiment of the invention the doctor means (11)and/or (11′) comprises at least one rotating cylindrical doctor rod.Rotating doctor rods are known in principle to the skilled worker. Theytypically have a diameter of a few cm, 1 to 3 cm for example, withoutany intention that the invention should be restricted to this. Therotating doctor rod is mounted rotatably in a suitable device and ispressed against the roll (4) and/or (4′), the exerted pressurepreferably being adjustable. The rotating doctor rod may be set inrotation through the rotation of the rolls (4), (4′), but preferably isactively driven. As a result of this it is possible to set the speeds ofthe rolls (4) and/or (4′) and also of the rotating doctor rod separatelyfrom one another, thereby allowing a certain slip to be set between theroll and the rotating doctor rod. The rotating doctor rod (1.1) and/or(11′) may run in the same direction or else—where actively driven—in theopposite direction of rotation to the rolls (4) and/or (4′). Therotating doctor rod is preferably closely adjacent to the applicatormeans (11). Examples of such arrangements are found in. U.S. Pat. No.2,970,564, DE 34 17 487 A1 or DE 37 35 889 A1, without any intentionthat the invention should be thereby restricted to those embodiments. Itis possible, for example, to arrange the rotating doctor rod such thatthe polymer formulation is fed from the top side directly into thecavity formed by applicator roll and rotating doctor rod, by pouring orspraying, for example.

The amount of the formulation that is intended to remain on theapplication rolls (4) and/or (4′) can be set via the nature of therotating doctor rod. The rotating doctor rod used preferably comprisesgrooves in the surface—that is, the surface of the rotating doctor rodis not planar but instead has indentations, such as indentations in lineor dot format. The nature of the grooves, such as, for example, thenature, depth, and density of the indentations, enables setting of theamount of formulation which is intended to remain on the roll. For thispurpose it is possible to hold in stock a plurality of differentrotating doctor rods, with different grooves, and to install them on thecoil coating line in accordance with the nature of the formulation usedfor passivating. Alternatively the rotating doctor rod can also be woundwith a wire, or may constitute a wire spiral.

The film located on the roll (4) and/or (4′), on contact with thegalvanized surface of the steel strip, is transferred wholly or partlyto the galvanized surface. Here, the amount of formulation located onthe application rolls (4) and/or (4′) per unit area is advantageouslysuch that no build-up, or at least no substantial build-up, is formedahead of the gap (12). The transfer of the film from the roll, (4)and/or (4′) to the galvanized surface should be very largely complete.The amount is further made such as to result in the film thicknessdesired for the specific application. It has been found appropriate touse an amount of less than 5 g/m² of the aqueous formulation, based onan approximately 20% formulation.

After the aqueous formulation has been applied, it reacts with thegalvanized surface, with formation of hydrogen. The acidic groups of thepolymer anchor the polymer to the surface, and the detached Zn²⁺ ionscrosslink the polymer coat.

The wet film, finally, is dried in a drying means (6) disposeddownstream of the pair of rolls in the direction of travel (2) of thestrip. In this context it is possible to make use, for example, offorced-air dryers or IR dryers. The drying temperature is set by theskilled worker in accordance with the formulation used and with thedesired properties of the coat. A temperature of 30 to 95° C. andpreferably 40 to 80° C. has been found appropriate, in each casemeasured as the peak metal temperature. The temperature of the forcedair in a forced-air dryer may of course also be higher.

The time from application of the formulation by the rolls (4) and/or(4′) to entry into the dryer, in other words the reaction time, can beinfluenced by the skilled worker, in a way which is known in principle,by the strip speed and/or by the distance of the dryer from the point ofapplication.

After passage through the dryer, a dried passivation coat remains on thegalvanized steel strip. The dry film thickness can be determined by theskilled worker via the amount of formulation applied, by means of themeasures already outlined. The film thickness is guided in each case bythe desired properties of the coat.

The apparatus used for the method of the invention may of course alsocomprise further components. Thus it is possible in principle for eachof the application rolls (4) and/or (4′) to possess more than one doctormeans (11). It is possible, furthermore, for further means for smoothingthe passivation coat to be provided between the application rolls (4),(4′) and the dryer—additional doctors and/or rolls, for example.

Using the method of the invention it is possible to achieve dry coatthicknesses of less than 1 μm, preferably less than 0.6 μm, and morepreferably 0.01 to 0.4 μm, which are nevertheless very uniform andexhibit a high corrosion resistance. On the basis of the highconcentration at which it is used, the polymer reacts very quickly, sothat there is virtually no longer any afterreaction of the acidicpolymer and hence also no longer any whitening of the passivation coat.

The examples below are intended to illustrate the invention in moredetail:

Polymer Solution Used:

The experiments were carried out using a 25% strength solution of acopolymer of 70% by weight acrylic acid and 30% by weightvinyiphosphonic acid in water.

COMPARATIVE EXAMPLE

For the comparative example a commercial laboratory squeeze-off devicewas used.

The device is composed of two counter-rotating steel rolls (totaldiameter 100 mm; length 350 mm) each provided with a rubber lining(Shore A hardness: 80). The rollers are pressed against one another witha linear pressure of approximately 10-15 N/mm, and rotated with a speedof approximately 25 m/min.

For the coating experiment, a commercial steel sheet for testing use(200 mm×100 mm) was immersed completely into the above solution forapproximately 1 s and then immediately squeezed off by means of thesqueeze-off device, the metal sheet being introduced into the gapbetween the two rolls, and so transported through the gap by therollers. The rollers had been wetted beforehand with the polymersolution. After squeezing off, the treated metal sheet is immediatelydried for a number of seconds in an oven at a PMT (peak metaltemperature) of 50° C.

The thickness of the polymer film on the metal after drying was 370 nm.

INVENTIVE EXAMPLES

For the inventive examples, a smooth rotating doctor rod (diameter: 12mm) was installed on the upper of the two rollers. The modified deviceis shown diagrammatically below.

The rotating doctor rod is held by a mount so that the pressure of therod on the roll can be altered. By virtue of its construction, therotating doctor rod is secured against sagging. By means of a meteringdevice, the polymer solution is fed from the top side directly into thegap between the rotating doctor rod and the press-off roll, in such away that there is always a small column of liquid between the rolls, butthe solution does not overflow. The pressure between the two rollers andalso their speed of rotation remain as above.

For coating, a galvanized steel sheet was, introduced into the gapbetween the two rollers, and so was transported through the gap by therollers. The metal sheet was then dried as above.

Three experiments were carried out with different pressures of therotating doctor rod. The results are compiled in the table below.

Example Pressure of rotating doctor rod Thickness of the polymer film 10.25 bar  90 nm 2 0.5 bar 40 nm 3 1.0 bar 30 nm

The examples show that, by means of the method of the invention, it ispossible to obtain significantly thinner polymer films than in the caseof the conventional method by direct application and squeezing-off ofthe excess. Furthermore, by means of doctor rods with different surfacestructuring, it is additionally possible to exert very precise controlwithin a wide range of film add-ons.

1. A method of continuously passivating a galvanized steel strip with anacidic aqueous formulation comprising at least one water-soluble polymerX comprising acidic groups, the polymer X having at least 0.6 mol ofacid groups/100 g of polymer, the pH of the formulation being not morethan 5, and the amount of all the polymers X together being 10% to 30%by weight, based on the amount of all the components of the formulation,and the galvanized steel strip being moved in its longitudinal directionthrough an adjustable gap between two opposing rolls, which rotate inthe direction of travel of the steel strip, and wherein the acidicaqueous formulation is applied to at least one of the two rolls using anapplicator means, and between the applicator means and the gap in thedirection of rotation of the roll there is at least one doctor meanswith which, by removal of excess formulation, it is possible to set theamount of the aqueous formulation per unit area of the roll, the filmlocated on the roll, on contact with the galvanized surface of the steelstrip, is transferred wholly or partly to the galvanized surface, andthe wet film is dried in a drying means disposed downstream of the pairof rolls in the direction of travel of the strip.
 2. The methodaccording to claim 1, wherein the doctor means comprises a meanscomprising at least one rotating doctor rod.
 3. The method according toclaim 2, wherein the rotating doctor rod has grooves in the surface. 4.The method according to claim 1, wherein the doctor means comprises ameans comprising at least one doctor knife.
 5. The method according toclaim 1, wherein formulation removed by the doctor means is returned tothe coating operation.
 6. The method according to claim 1, wherein theamount of the aqueous formulation per unit area of the roll is set suchthat no build-up is formed ahead of the gap.
 7. The method according toclaim 1, wherein the thickness of the dried passivation coat formed isless than 1 μm.
 8. The method according to claim 1, wherein the acidicgroups of the polymer X are groups selected from the group consisting ofcarboxyl groups, sulfonic acid groups, phosphoric acid groups, andphosphonic acid groups.
 9. The method according to claim 1, wherein theacidic groups of the polymer X are groups selected from the groupconsisting of carboxyl groups, phosphoric acid groups, and phosphonicacid groups.
 10. The method according claim 1, wherein the acidic,water-soluble polymer X is a homopolymer or copolymer comprising(meth)acrylic acid units.
 11. The method according to claim 1, whereinthe acidic, water-soluble polymer X is a copolymer X1 which is composedof the following monomeric units—based in each case on the amount of allthe monomers copolymerized into the copolymer: (A) 30% to 90% by weightof (meth)acrylic acid, (B) 10% to 70% by weight of at least one furthermonoethylenically unsaturated monomer, different from (A), which has oneor more acidic groups, and also (C) optionally 0% to 40% by weight of atleast one (meth)acrylic ester having OH groups, (D) optionally 0% to 30%by weight of at least one further ethylenically unsaturated monomer,different from (A), (B) or (C).
 12. The method according to claim 11,wherein the monomers (B) are monomers selected from the group consistingof (B1) monoethylenically unsaturated dicarboxylic acids having 20 to 23carbon atoms, and/or (B2) monoethylenically unsaturated phosphoricand/or phosphonic acids.
 13. The method according to claim 1, whereinthe amount of monomer (C) is 1% to 30% by weight.
 14. The methodaccording to claim 1, wherein not more than 25 mol % of the acid groupsof the polymer X are neutralized.
 15. The method according to claim 1,wherein the amount of acids in the formulation does not exceed 20% byweight relative to the amount of all the polymers X together. 16.(canceled)
 17. The method according to claim 2, wherein formulationremoved by the doctor means is returned to the coating operation. 18.The method according to claim 3, wherein formulation removed by thedoctor means is returned to the coating operation.
 19. The methodaccording to claim 4, wherein formulation removed by the doctor means isreturned to the coating operation.
 20. The method according to claim 2,wherein the amount of the aqueous formulation per unit area of the rollis set such that no build-up is formed ahead of the gap.
 21. The methodaccording to claim 3, wherein the amount of the aqueous formulation perunit area of the roll is set such that no build-up is formed ahead ofthe gap.